Conventionally, cleaning of silicon substrates, and glass substrates, which are to be used for semiconductor devices and liquid crystal devices, respectively, and other types of substrate has been primarily conducted by the so-called RCA cleaning method. By this method, these substrates are cleaned at a high temperature using a concentrated hydrogen peroxide based chemical solution such as a mixture of hydrogen peroxide solution and sulfuric acid; a mixture of hydrogen peroxide solution, hydrochloric acid, and water; or a mixture of hydrogen peroxide solution, ammonia solution, and water, and then rinsed the substrates with ultrapure water. However, this RCA cleaning method uses large quantities of hydrogen peroxide solution, highly concentrated acids, highly concentrated alkalies, and the like, thus the costs for chemical solutions are high. Additionally, ultrapure water for rinsing, waste liquid treatment, ventilation, i.e., discharging chemical vapor and preparing clean air and the like entail a lot of costs.
In view of such situation, various measures intended to reduce costs in the cleaning process and to lessen the adverse impact on the environment have been taken and in this regard several achievements have been made. A typical example of such an achievement is a technique for cleaning a treated object by ultrasonic cleaning and the like using water containing dissolved gas in which a specific gas is dissolved. The specific gas may be an oxygen gas, ozone, carbon dioxide gas, a rare gas, an inert gas or a hydrogen gas.
As a process for producing such water containing dissolved gas, a process using a film module with a built-in gas permeable film is known. In the process, water is fed to the liquid-phase side of the gas permeable film and a specific gas is supplied to the gasphase side of the gas permeable film. By permeating the gas permeable film, the gas in the gasphase side dissolves into the water in the liquid-phase side, and thus water containing dissolved gas is produced.
For example, Japanese Patent Publication 11-077023A describes a process of deaerating ultrapure water thus reducing the saturation degree of dissolved gas, and then dissolving hydrogen gas into the ultrapure water.
FIG. 2 is a process flow diagram illustrated in the publication mentioned above. Ultrapure water is fed through a flow meter 1 to a deaeration film module 2. In the deaeration film module 2, the gasphase side, which is separated from the ultrapure water by a gas permeable film, is kept in a decompressed state with a vacuum pump 3, thus gas dissolved in the ultrapure water is deaerated. The ultrapure water with its dissolved gas deaerated is then fed to a hydrogen gas dissolution film module 4. In the hydrogen gas dissolution film module 4, hydrogen gas supplied from a hydrogen gas feeder 5 is injected into the gas phase side and is fed to the ultrapure water through a gas permeable film. To the ultrapure water having dissolved hydrogen gas that has a desired concentration is added a chemical solution such as ammonia water conveyed from a chemical solution storage tank 6 with a chemical feeding pump 7, and the pH is adjusted to a predetermined value. Hydrogen gas is dissolved, and the alkaline turned hydrogen containing ultrapure water is finally fed to a precision filtration device 8 where fine particles are removed with an MF filter or the like.
With dissolved gas measurement sensors 9 installed at the inlet port and outlet port of the deaeration film module 2, the amounts of gas in the ultrapure water are measured and the saturation degrees are determined. Signals are sent to the vacuum pump, the saturation degrees of the ultrapure water are compared with the desired saturation degree, and the deaeration amount is adjusted. The adjustment of the deaeration amount is conducted by, for example, controlling the degree of vacuum by fixing the aperture of the vacuum degree controlling valve. The gas saturation degree of the ultrapure water after the deaeration is measured with dissolved gas measurement sensors 9, and the hydrogen gas concentration in the hydrogen containing ultrapure water that has flowed out of the hydrogen gas dissolution film module is measured with a dissolved hydrogen measurement sensor 9A. Signals representing these measurements are sent to the hydrogen gas feeder, and the feed amount of hydrogen gas is controlled by, for example, fixing the aperture of the valve installed in the feed route of hydrogen gas.